The processes for fabricating high resolution patterns, mainly on planar objects, by selective etching or deposition has been well known for centuries. In general, the layer to be shaped or patterned is covered by a protective layer known as a "resist". The desired shapes are created in the protective layer, usually via photo-imaging. The exposed (or unexposed, if the resist is negative working) part of the image is removed, normally by using a liquid developer to expose the layer underneath. The exposed layer can now be etched through the openings in the resist layer, which protects the covered area from the etching process. Etching can be by wet chemicals or by dry plasma (a process widely used in the semiconductor industry). Instead of etching an additive process can be used. In additive process a material is deposited through the openings in the resist to add to the layer underneath the resist. This deposition can be done in a wet process (as in the well known "additive" process for manufacturing printed circuit boards) in a dry process, such as a vacuum deposition by evaporation or sputtering. Another way of using a resist is in allowing chemical reactions, such as oxidation, to occur only in the areas not covered by the resist.
In general, a resist is an imagewise mask selectively controlling a chemical or physical process and limiting the process to follow the image pattern. The term "resist" should be interpreted in this broad sense throughout this disclosure and claims. At the end of the process the remaining resist is normally removed, or "stripped". Historically most resists were photoresists, i.e. activated and imaged by the photonic action of the light. Because of this photonic action most photoresists operate in the UV part of the spectrum, where the photon energy is high. Some resists can be exposed by other types of radiation, such as electron-beams. All photoresists and electron-beam resists share one fundamental property: they respond to the total exposure, not to the momentary illumination. In optics, exposure is defined as the integral of illumination over time. For example, a photoresist can be exposed by 100 mW/cm.sup.2 for 1 sec to yield 100 mJ/cm.sup.2 (100 mw.times.1 sec) or it can be exposed by 1000 mW for 0.1 sec with similar results. (1000 mW.times.0.1 sec=100 mJ/cm.sup.2). This law is also known as "reciprocity law" and it is the basic law governing the exposure of photoresists. When a certain exposure is reached, a change occurs in the resist. The most common resists operate by a change of solubility in a developer.
The law of reciprocity also requires a high contrast ratio and low stray light in optical systems used to exposure photoresists and electron beam resists. For example, if an exposure system has a light leakage, or stray light, of 1% (e.g.: when exposure is "off", the light level does not drop to zero but only to 1% of the "on" state) the effect of this stray light can be as large (or larger) than the main exposure if left on the photoresist for a long time.
An even larger problem is caused when trying to image high resolution features: the point spread function of the optical system causes a "spreading" of light from each feature. This causes light from on feature to overlap with adjacent features and lower the resolution. This is shown in FIG. 1. Feature 1 has a light distribution 1' and feature 3 has a light distribution 3'. Exposure curve 2, generated by lens 8 imaging feature 1, is added up to exposure curve 4, generated by lens 8 imaging feature 3, to create a curve 5, which is the equivalent exposure. Curve 5 creates a distorted image 6 and 7 of features 1 and 3 on photoresist 9 having a threshold 10. It is important to understand that it makes no difference if exposures 2 and 4 are applied simultaneously or sequentially, as the photoresist will add up, or integrate, the exposures.
Recently a different type of resist, known as thermoresist, has been used in the manufacturing of printing plates and printed circuit boards. A thermoresist (also known as a thermal resist or heat-mode resist) changes solubility when a certain temperature, rather than a certain accumulated exposure, has been reached. Such thermoresists are imaged using near infra-red light and therefore are also known as "IR resists". Some examples of thermoresists are disclosed in the following U.S. Pat. Nos.: 5,340,699 (Haley); 5,372,907 (Haley); 5,372,915 (Haley); 5,466,557 (Haley); 5,512,418 (Ma); 5,641,608 (Grunwald); 5,182,188 (Cole); 5,314,785 (Vogel) and 5,328,811 (Brestel). The thermoresist described by Haley is unusual as the same composition acts as a photoresist, obeying the reciprocity law, when exposed by UV light (at low power density) but also acts as a thermoresist, responding only to temperature, when heated up by IR at high power density. Thermal resist is also available from Creo Ltd. (Lod Industrial Park, Israel), sold under the trade name "Difine 4LF". All of the above-mentioned thermoresists respond to temperature and do not follow the reciprocity law. It is not possible to have a practical true thermoresist which follows the reciprocity law. Such a thermoresist would be exposure simply by long exposed to ambient temperature (just as a photoresist will get exposed by a long exposure to ambient light). While it is possible to shield a photoresist from ambient light it is not possible to shield from ambient temperature. Therefore a practical thermoresist cannot obey the reciprocity law. Prolonged exposure to ambient temperature below the threshold temperature has little effect on a thermoresist. Obviously, the threshold temperature needs to be well above the temperature expected to be encountered in shipping and storage. When chemical reaction in a thermoresist does not have a sharp threshold temperature, the chemical composition has to be formulated to keep the reaction rate very low at room temperature. This is not difficult to do, as most chemical reaction rates approximately double every 10 degrees centigrade. Thus the reaction rate in a thermoresist exposed at 350 degrees centigrade can be a billion times faster than at 25 degrees. Using lasers it is fairly easy to raise the temperature of a thermoresist to over 1000 degrees. Such a thermoresist will appear to have a distinct threshold simply because the reaction rate at lower temperature slows down exponentially. To follow the reciprocity law the reaction rate would have to change in a linear fashion with temperature.
Light valves, also known as multi-channel modulators or spatial light modulators, break up a single light beam into a linear or two-dimensional array of individually addressable spots. An example of using light valves to expose photoresists is shown in U.S. Pat Nos. 5,208,818 (Gelbart) and 5,296,891 (Vogt). Multi-beam, also known as multi-spot, scanning is well known in the art and is used to increase writing speed by exposing a plurality of features simultaneously. The limiting factor in both these patents is the leakage light from the light valves used. Even if the light valves were ideal, the limited optical resolution of the imaging lens creates a problem equivalent to stray light as previously explained.
It is an object of the invention to expose a resist layer using low contrast optical systems, and in particular low contrast light valves. This invention is enabled because thermoresists violate the reciprocity law. Such thermoresists do not integrate the exposure, and any stray heat will dissipate quickly. It is therefore possible to image thermoresists using low contrast (i.e. high light leakage) light valves. Another object of the invention is to increase the resolution achievable in optical lithography by combining the unique properties of thermoresists with the short wavelength of UV light. A further object is to image articles such as integrated circuits, flat panel displays and printed circuit boards without the use of phototools. Phototools is a generic name used to describe the films or glass masks currently used as a master for imaging photoresists via contact or optical projection. Still a further object of the invention is to overcome the shallow depth of focus of today's high resolution exposure machines used to expose integrated circuits by using multiple exposures of thermoresist. These and other objects of the invention will become apparent by considering the following description in conjunction with the drawings.
Low contrast optics, including low contrast linear light valves can be used to create high resolution patterns by using thermoresist instead of photoresist and by using multiple exposures of the same area, preferably exposing different features of the pattern in each exposure. Since thermoresists respond to temperature, the stray light from imaging individual features does not add up as the stray heat it creates will dissipate between exposures. The method is particularly useful for imaging thermoresists using UV light for manufacturing of integrated circuits.